1. Technical Field
The invention relates in general to the field of image compression and, more particularly, reconstructing previously compressed black and white images.
2. Description of the Related Art
As the popularity of the Internet continues to grow, an increasing number of individuals make various types of information available over the Internet. This information may include various types of documents with a combination of pictures and text. To provide this information, these individuals may post a compressed version of the original image on a web site on the World Wide Web.
The Joint Photographic Experts Group (JPEG) designed one of the most common methods for compressing images. JPEG is a compression method that uses a Discrete Cosine Transform (DCT). This method effectively compresses photographic images. Though JPEG may be used for photographic images, its design impedes efficient compression of bi-level, or black and white, images.
To save time, some individuals compress documents containing graphics and text using the JPEG method. Alternatively, some individuals choose JPEG compression for a completely textual image because of its availability. Regardless of the reason, using JPEG to compress a bi-level image introduces significant error into the image""s compressed representation. Although JPEG compression normally causes loss of some data pertaining to an original image, use of such compression technique on a bi-level image can increase the amount of data loss so significantly that it may render the decompressed image unusable.
Previously, improving the quality of a JPEG-compressed bi-level image in the absence of an original image appeared impractical. As many websites and desktop publishing software utilize JPEG images, there is a need for a method for generating a higher quality image.
The present invention satisfies the above-mentioned need in a method for generating a higher quality reconstructed image from a lower quality image resulting from JPEG compression of a bi-level image. The invented method comprises thresholding to force pixels of a starting image to be closer to bi-level to generate a threshold image, transforming the threshold image to generate transform coefficients corresponding to a decomposition of the threshold image, selectively clamping the transform coefficients into quantization bins defined by compression of the bi-level image, and applying an inverse-transform on the modified coefficients to generate the higher quality reconstructed image. The starting image and the reconstructed image can be compared to determine the degree of improvement obtained with the method. The method can be repeated iteratively to obtain an image that more closely represents the original image than does the original lower-quality image.